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Evolution = Descent with Modification Species originate as modified descendants of other species. Biological evolution is the origin of new species from previously existing ones over time.

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“As many more individuals of each species are born than can possibly survive, and as consequently there is a frequently recurring struggle for existence, it follows that any being, if it vary in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of survival and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.” Charles Darwin, The Origin of Species

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Darwin’s Theory If there is a struggle for existence… If heritable variations exist among members of a population… If these variations affect survival and/or reproduction… Then species will change over time.

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There is a struggle for existence. Species tend to produce more offspring than the environment can support. Darwin calculated that even one slow-breeding female elephant could give rise to 19,000,000 elephants after 750 years if all of her young survived and reproduced at the same rate. Obviously, the environment cannot support all elephants that are born.

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Heritable variations exist. An impressive amount of variation exists within living species. Breeders of animals and plants use these diverse characteristics to establish new varieties of dogs, cats, pigeons, wheat, cotton, corn, and other domesticated organisms.

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Populations evolve, not individuals. Individuals may change throughout their lifetimes, but this is not evolution. Evolution occurs across many generations. Over time, populations acquire new characteristics that enhance their ability to survive and/or reproduce in a given environment.

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Variations are “Random” Mutation and sexual recombination occur randomly. The variations that occur do not anticipate the “needs” of the organism. In this sense, “random” does not mean that anything can happen.

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Natural Selection is NOT Random Genetic variation occurs randomly, but natural selection is NOT random. Genetic variations that aid survival and reproduction are more likely to become common than variations that do not. Nature imposes a direction to evolutionary development. Though dolphins are more closely related to humans than they are to sharks, they have evolved bodies adapted to an aquatic environment.

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Evolution is About Consequences Evolution is historically contingent, that is, “windows of opportunity” constantly open and close. Once certain changes have occurred, the range of possible future changes is permanently altered.

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“Windows of Opportunity” The earliest turtles could not retract their necks. Proganochelys 220 million years ago Proganochelys 220 million years ago side neck turtles (southern hemisphere) side neck turtles (southern hemisphere) vertical neck turtles (northern hemisphere) vertical neck turtles (northern hemisphere) Later, two distinct forms of neck retraction evolved. With the extinction of the turtles that could not retract their necks, the possibility that another form of neck retraction would evolve was lost.

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Evolution is Testable Human & Ape Chromosome Numbers Hypothesis: Human chromosome 2 resulted from the fusion of two chromosomes present in the common ancestor of humans and the other great apes. 1958 Human chromosome number is 46 1960 Chimpanzee chromosome number is 48. 1961 Gorilla and orangutan chromosome numbers are 48. 1975 Detailed analysis confirms that four species share ancestry. 1980 Banding patterns of humans and chimpanzees match. 1982 Telomeres in the middle of human chromosome 2. centromere telomeres

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Evolution is Testable Human & Ape Chromosome Numbers These scientists were not limited to simply noting the similarities in the chromosomes. They were able to use evolutionary theory to consistently predict what they would later confirm to be correct. Video

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Is Evolution a Theory or a Fact? Evolution is both a fact and a theory. Scientists no longer really question the basic fact that evolution has occurred. The concept of common descent has withstood nearly 150 years of extensive testing. There are various competing theories used to explain HOW evolution happens.

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Is Evolution Controversial? The socio-political controversy stems from the notion science is trying to explain phenomena that some religions have already explained. The genuine scientific disagreements concern technical points about how evolution happens, not whether it has occurred.